Indirect evaporation-based solutions for humidification-dehumidification desalination processes

This thesis focuses on studying the mechanisms of water–humid air interaction to theorise and develop proposals for novel Humidification-Dehumidification (HDH) desalination schemes. In particular, it revisits the Maisotsenko thermodynamic cycle (M-cycle), typically used in Indirect Evaporative Cooling (IEC) applications, as a key component for desalination purposes. The research investigates two main aspects: first, the water-humid air interaction occurring in M-cycle devices, in terms of mass and energy transfer; second, the modelling of innovative HDH cycle proposals. These proposals are named “hybrid” schemes, as they integrate traditional HDH designs with IEC technology and other technologies, such as the Vapour Compression Refrigeration (VCR) cycle and Waste Heat (WH) recovery. The study addresses heat and mass transfer phenomena using a one-dimensional numerical approach. The modelling has been calibrated against experimental tests on a commercial device produced by an established Manufacturer which collaborates with the PhD Candidate’s Research Group, in order to define proper values for the Nusselt number (which defines heat and mass transfer coefficients) and to extend the standard operative range of the reference commercial device. The development of HDH schemes is conducted using zero-dimensional numerical methods, incorporating results from both the general one-dimensional model and commercial machines to give an applicative target to the study. HDH proposals have been compared in terms of energy consumption (Specific Energy Consumption - SEC indicator) and water recovery (Recovered Water-to-Air Ratio - RWA indicator). The main findings of this thesis can be summarised as follows. Regarding the M-cycle IEC modelling: a Nusselt number of 7.0, equal for dry and wet channels, makes the modelling consistent with reality; some correlations have been derived to adapt the model to a larger operative range. Regarding the HDH proposals: the scheme IEC + VCR has SECel = 137.3 kWh/m3, RWA = 6.2 ∙ 10-6 ; the scheme IEC + WH has SECel = 21.3 kWh/m3 , SECth = 841.6 kWh/m3 , RWA = 16.3 ∙ 10-6. Parametric analyses indicate that the proposed designs exhibit particularly promising desalination efficiencies, positioning them as highly competitive within the HDH sector and opening exciting prospects in this field.